Fix: "isAlias" of CodegenModel (#2758)

* Add test case to reproduce the issue

https://github.com/OpenAPITools/openapi-generator/issues/2574

* Fix: an alias of "an alias of simple OAS type" has an incorrect property `isAlias: false`

* Use ModelUtils instead of referring the "type" value directly

* Delete an unnecessary condition

* Tweak: the order of conditions

* Fix wrong "isAlias" value on ComposedSchema

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

@@ -2047,7 +2047,8 @@ public class DefaultCodegen implements CodegenConfig {
         if (schema.getExtensions() != null && !schema.getExtensions().isEmpty()) {
             m.getVendorExtensions().putAll(schema.getExtensions());
         }
-        m.isAlias = typeAliases.containsKey(name);
+        m.isAlias = (typeAliases.containsKey(name)
+                        || isAliasOfSimpleTypes(schema)); // check if the unaliased schema is an alias of simple OAS types
         m.discriminator = createDiscriminator(name, schema);
 
         if (schema.getXml() != null) {
@@ -4204,11 +4205,10 @@ public class DefaultCodegen implements CodegenConfig {
 
         Map<String, String> aliases = new HashMap<>();
         for (Map.Entry<String, Schema> entry : schemas.entrySet()) {
-            String oasName = entry.getKey();
             Schema schema = entry.getValue();
+            if (isAliasOfSimpleTypes(schema)) {
+                String oasName = entry.getKey();
                 String schemaType = getPrimitiveType(schema);
-            if (schemaType != null && !schemaType.equals("object") && !schemaType.equals("array")
-                    && schema.getEnum() == null && !ModelUtils.isMapSchema(schema)) {
                 aliases.put(oasName, schemaType);
             }
 
@@ -4217,6 +4217,14 @@ public class DefaultCodegen implements CodegenConfig {
         return aliases;
     }
 
+    private static Boolean isAliasOfSimpleTypes(Schema schema) {
+        return (!ModelUtils.isObjectSchema(schema)
+                && !ModelUtils.isArraySchema(schema)
+                && !ModelUtils.isMapSchema(schema)
+                && !ModelUtils.isComposedSchema(schema)
+                && schema.getEnum() == null);
+    }
+
     /**
      * Remove characters not suitable for variable or method name from the input and camelize it
      *

modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultCodegenTest.java

@@ -950,6 +950,28 @@ public class DefaultCodegenTest {
         Assert.assertTrue(cm.isDouble);
     }
 
+    @Test
+    public void testAlias() {
+        final OpenAPI openAPI = TestUtils.parseSpec("src/test/resources/3_0/type_alias.yaml");
+        new InlineModelResolver().flatten(openAPI);
+
+        final DefaultCodegen codegen = new DefaultCodegen();
+        codegen.setOpenAPI(openAPI);
+
+        CodegenModel typeAliasModel = codegen.fromModel(
+                "MyParameterTextField",
+                openAPI.getComponents().getSchemas().get("MyParameterTextField")
+        );
+        Assert.assertTrue(typeAliasModel.isAlias);
+        Assert.assertEquals("string", typeAliasModel.dataType);
+
+        CodegenModel composedModel = codegen.fromModel(
+                "ComposedModel",
+                openAPI.getComponents().getSchemas().get("ComposedModel")
+        );
+        Assert.assertFalse(composedModel.isAlias);
+    }
+
     private void verifyPersonDiscriminator(CodegenDiscriminator discriminator) {
         CodegenDiscriminator test = new CodegenDiscriminator();
         test.setPropertyName("DollarUnderscoretype");

modules/openapi-generator/src/test/resources/3_0/type_alias.yaml

@@ -0,0 +1,44 @@
+openapi: "3.0.1"
+info:
+  version: 0.0.1
+  title: broken API
+paths:
+  /test:
+    put:
+      summary: No description
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/MyParameter'
+      responses:
+        200:
+          description: "OK"
+components:
+  schemas:
+    MyParameter:
+      type: object
+      properties:
+        text_field:
+          $ref: '#/components/schemas/MyParameterTextField'
+    MyParameterTextField:
+      $ref: '#/components/schemas/TypeAliasToString'
+    TypeAliasToString:
+      type: string
+      minLength: 1
+      maxLength: 50
+    BaseModel:
+      type: object
+      discriminator: className
+      required:
+        - className
+      properties:
+        className:
+          type: string
+    ComposedModel:
+      allOf:
+        - $ref: '#/components/schemas/BaseModel'
+        - type: object
+          properties:
+            testProperty:
+              type: string